The invention relates to a process for the concentration and dehydration of sewage sludge, of sewage sludge in particular, which contains activated sludge produced in an aerated biological purification stage.
In industrial and/or local biologically aerobic sewage water purification it is the common desire to produce a compact sewage sludge of easy filtrability and high dry substance content in the filter cake. The reduction of sludge volume accompanying a high dry substance content, due to separation of the sewage water portion, allows for economical and energy-saving further usage of the sewage sludge by storing it on waste dumps, putrefication, combustion or use in agriculture.
Usually, process technological usability of the activated sludge in the sewage treatement plant is determined in that a sample is taken from the activated pond and the precipitation volume thereof is visually judged in the measuring cylinder.
The goal to be achieved in any biological sewage treatment plant always is a precipitation volume being as low as possible, of the activated sludge. This facilitates the process technology to be applied for clarification, since low sludge precipitation volumes always are equivalent to low consumption of inorganic and/or organic flocculators.
With respect thereto, from EP-A-0 355 189 an improved procedure for concentrating and dehydrating sewage sludge using reduced amounts of organic and/or inorganic flocculators is known, so that in an aerated biological purification stage using a water-soluble preparation which contains folic acid and/or dihydrofolic acid and/or at least one ammonium, alkali metal, alkaline earth metal and/or alkanol ammonium salts thereof, an activated sludge with quick precipitation having low precipitation volume is produced.
The kind of procedure under EP-A-0 355 189 has already been introduced into practice successfully (S. J. Ostrander in: Operations Forum, vol. 9, no. 1, 1992, pages 10 to 13).
Stable folic acid compositions are described in EP-A-0 319 598, too.
It is, however, a disadvantage of the procedural method of this patent that the effect of the improved sludge dehydration of the activated sludge ocurrs with temporal delay. Generally, several weeks of fore-runnings in adding folic acid/dihydrofolic acid are required before the effect occurs.
If the waste water originating from the pre-purification and entering the biological purification stage moreover in addition contains toxic substances, the beginning of the improved precipitation behavior of the activated sludge under the method of EP-A-0 355 189 is deferred to about 60 to more than 120 operating days.
In laboratory tests a fore-running period of 5 to 10 days could be achieved, therein, however, no toxic matter usually always contained in the waste water flowing into the activated pond having been added.
For this reason it was tried to shorten the period up to the starting of the improved precipitation behavior by means of increased added amounts of the preparation as defined in the above-stated Letters Patent. The results obtained, however, were not satisfactory and the economy of the procedural method was not guaranteed due to the high dosing of folic acid and/or dihydrofolic acid.
The use of water-soluble molybdenum compounds as micronutrients and as additives for nutrient substrates is known in biotechnology, e.g. from G. Drews: "Mikrobiologisches Praktikum", Berlin 1983, page 11.
U.S. Pat. No. 4,429,043 teaches the use of water-soluble molybdenum compounds as micronutrients, in addition to iron and copper, for an anearobic sewage sludge treatment for preparation of methane gas. In this anaerobic process the sewage sludge, however, is not produced in the presence of folic acid and/or dihydrofolic acid. Nor is there any hint towards to an improved precipitation behavior of the digested sludge.
It is known to the expert in the field of sewage plant technology that the sewage water flowing into the biological purification stage already may contain up to 0.02 ppm of molybdenum in dissolved form, without a synergetic effect on the precipitation behavior of the activated sludge being known when carrying out the process as described in the patent specification of EP-A-0 355 189 (see also L. Hartmann, "Biologische Abwasserreinigung", Berlin 1989, page 188).
Tests in which in two separate partial streams, on one hand folic acid compounds and on the other hand water-soluble molybdenum compounds were added to the waste water flowing into the biological purification stage did not produce results, i.e. a significant reduction of response time up to the start of improved precipitation behavior of the activated sludge could not be achieved, even when large amounts of folic acid and molybdenum were added in separated partial streams.